Security readout for executives and security teams
Plain-English summary
This is a Linux kernel privilege-escalation flaw. A local user could abuse an io_uring buffer-length handling bug to corrupt kernel memory and potentially run code with kernel privileges. It is serious for shared Linux systems, but the provided sources do not show confirmed active exploitation.
Executive priority
Patch on a high-priority operational timeline, especially for shared servers and environments with untrusted local execution. This is not evidenced as actively exploited in the supplied sources, but kernel-level code execution makes delayed remediation materially risky.
Technical view
CVE-2021-3491 affects Linux kernel io_uring PROVIDE_BUFFERS handling. Lengths above MAX_RW_COUNT were not truncated, allowing negative values in mem_rw during /proc/<PID>/mem reads and leading to a heap overflow. The issue was introduced in v5.7-rc1 and fixed by commit d1f82808877b with stable backports.
Likely exposure
Exposure is most likely on Linux kernels from v5.7-rc1 through unfixed 5.10.y, 5.11.y, and 5.12.y builds. Systems are most at risk where low-privileged local users or untrusted workloads can run code. Vendor-backported kernels require advisory or package verification.
Exploitation context
The CVSS vector requires local access and low privileges, with high attack complexity and no user interaction. Successful exploitation could compromise confidentiality, integrity, and availability at kernel scope. The bundle marks CISA KEV as false and provides no cited evidence of active exploitation.
Researcher notes
Key evidence is consistent: io_uring PROVIDE_BUFFERS failed to cap lengths at MAX_RW_COUNT, enabling a kernel heap overflow path through /proc/<PID>/mem handling. The source bundle identifies fix commit d1f82808877b and stable backports; exploit availability is not established here.
Mitigation direction
- Update to vendor kernels containing the CVE-2021-3491 fix or equivalent backport.
- Use at least Linux 5.10.37, 5.11.21, 5.12.4, or later fixed vendor builds.
- Reboot systems after kernel updates so the patched kernel is running.
- Prioritize shared hosts and systems allowing low-privileged interactive or workload execution.
- If patching is delayed, check vendor advisories for supported temporary guidance.
Validation and detection
- Inventory running kernel versions across Linux assets.
- Compare versions against fixed stable releases and vendor advisory status.
- Confirm package changelogs or advisories mention CVE-2021-3491 or commit d1f82808877b.
- Verify systems have rebooted into the patched kernel after update.
- Review exposure where untrusted local users or workloads can execute code.
Public sources used
Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.
Conservative CVE-to-ATT&CK context
These mappings and lookup hints may be relevant to the vulnerability behavior, CWE, affected product, or exposure path. Glexia-inferred context is not an official MITRE, ATT&CK, CWE, or CVE Program mapping.
ATT&CK lookup starting points
Use these exact CWE pages and searches to review the Glexia ATT&CK library from this CVE's weakness and description context.
CWE-131: Exact CWE lookup
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Open ATT&CK lookupExecution behavior lookup
The CVE wording references code or command execution, so execution technique review may help defensive triage. This is a Glexia inferred lookup path, not an official MITRE, ATT&CK, or CVE Program mapping.
Open ATT&CK lookupCVE-2021-3491 mapping review
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Open ATT&CK lookup- Severity
- High
- CVSS
- 7.8 (3.1)
- Known Exploited
- No
- Published
Vector: CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:C/C:H/I:H/A:H
CNA and ADP enrichment extracted from CVE v5
These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.
CVSS vector scores
1 official scoreWe collect every scored CVSS vector available in the official CNA and ADP containers. When more than one version is present, the table keeps the source vectors side by side instead of collapsing them into the highest score.
CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:C/C:H/I:H/A:H1.16Primary CVE scoreVulnerability scoring details
Base CVSS 3.1 score
7.8HighVector: CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:C/C:H/I:H/A:H
Source materials
- CVE List V5 sourceCVE List V5
- https://ubuntu.com/security/notices/USN-4950-1CVE reference · vendor-advisory, x_refsource_UBUNTU
- https://ubuntu.com/security/notices/USN-4949-1CVE reference · vendor-advisory, x_refsource_UBUNTU
- [oss-security] CVE-2021-3491 - Linux kernel io_uring PROVIDE_BUFFERS MAX_RW_COUNT bypassCVE reference · mailing-list, x_refsource_MLIST
- https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=d1f82808877bb10d3deee7cf3374a4eb3fb582dbCVE reference · x_refsource_MISC
- https://www.zerodayinitiative.com/advisories/ZDI-21-589/CVE reference · x_refsource_MISC
- https://security.netapp.com/advisory/ntap-20210716-0004/CVE reference · x_refsource_CONFIRM
Products and packages named in the record
CWE details
CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.
Incorrect Calculation of Buffer Size
Incorrect Calculation of Buffer Size represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.